A variety of tools and machines have been proposed and/or are in use for tensioning a loop of strap around an article or articles, such as a stack of lumber, equipment on a pallet, and the like. Many such machines and tools also typically apply a deformable seal to secure the overlapping strap lengths together and then sever the trailing portion of the strap from a supply of the strap on a reel.
Typically, conventional strapping machines and tools of this type operate to grip a leading, free end segment of the strap with a suitable gripping device and then apply tension with a traction wheel which is rotated against the strap. After sufficiently high tension has been pulled on the strap, the tension is maintained on the strap while an open, generally C-shaped or U-shaped seal, which has been supplied from a magazine, is crimped about the overlapping strap portions to hold them together in tight engagement. At the termination of the crimping step, the trailing portion of the strap is severed by a suitable mechanism.
Conventional strapping tools of the type described above have been marketed in the U.S.A. by Signode Corporation, 3600 West Lake Avenue, Glenview, Ill. 60025. One such tool is marketed under the designation .div.SIGNODE Model ASD Combination Strapping Tool" and is disclosed in the "OPERATION AND PARTS MANUAL" for that tool as published by Signode Corporation under the document designation "REB 7/77-1M-A". Another such machine is marketed under the designation "SIGNODE AM COMBINATION STRAPPING TOOL" and is disclosed in the "OPERATION, PARTS AND SAFETY MANUAL" for that tool as published by Signode Corporation under the document designation "186027 (p.20E) Rev. 10-89". Other tools of this general type have been marketed under the designation "SIGNODE ASL and ASM COMBINATION STRAPPING TOOLS" and are disclosed in the "OPERATION, PARTS AND SAFETY MANUAL" for such tools as published by Signode Corporation under the document designation 186101 (p. 69A) Rev. 2-90".
The above-identified tools are manually operated and typically include a housing, a tensioning assembly, a seal magazine assembly, a sealer assembly for applying the seal to the overlapping lengths of the strap after the strap has been tensioned, and a cutter mechanism for severing the sealed loop from the trailing portion of strap. Other tools performing the same functions may be pneumatically or electrically operated. Further, the functions may also be incorporated in large, automatic machines which also operate to initially feed the strap around the article to be bound and form a loop which is subsequently tensioned, sealed, and severed from the supply of strap.
The above-identified types of tools and machines typically employ a pair of pivoting jaws for crimping the seal about the overlapping lengths of strap. Typically, the seal is steel, and the strap may be steel or a thermoplastic material. In either case, the jaws pivot to a closed position to deform the steel seal tightly about the overlapping strap lengths.
Different types of steel seals have been developed over the years for use with thermoplastic strap. Some seals are provided with teeth, such as disclosed in the Signode Corporation U.S. Pat. No. 3,636,592. The tooth structure disclosed in that patent has been incorporated in a commercially available seal marketed in the U.S.A. under the designation "50 ASD" by Signode Corporation, 3600 West Lake Avenue, Glenview, Ill. 60025. The 50 ASD seal has been widely used for a long time with the "softer" thermoplastic strap materials, such as nylon and polypropylene.
Although the 50 ASD seal functions well with nylon and polypropylene thermoplastic strap, it is not sold for use with the harder polyester thermoplastic strap. It has been found that relatively thin, sliver-like teeth, such as are used in the 50 ASD seal, tend to bend and deform when the seal is closed about the overlapping portions of polyester strap. Thus, many of the teeth in the seal may not function particularly well to penetrate and grip the strap material. A seal with such teeth would not provide as great a joint strength as would be desired in some applications when used on polyester strap.
If the overlapping strap lengths are not sufficiently restrained by the seal teeth, then the overlapping straps may slip within the seal, and the strap loop will become loose. Indeed, the joint may not be initially tight enough, or the joint may loosen with time and/or upon being subjected to external loading conditions such as vibrations or impacts during handling. This problem is further exacerbated by the relatively low friction coefficients exhibited by thermoplastic strap.
The surface of plastic strap may be relatively smooth and have a relatively low coefficient of sliding friction. In addition, plastic strap has a tendency to stretch and undergo a transverse reduction in the width dimension when subjected to substantial tensile forces over a period of time. Obviously, these characteristics can decrease the joint strength capability or integrity of a joint formed with a compressed seal at given compression force.
Some straps may be specially treated or coated (e.g., with wax or other materials) to improve appearance, improve automatic feeding characteristics within automatic strapping machines, or for other reasons. Such treated straps may have a lower coefficient of sliding friction than untreated strap and may slip more easily in a joint formed by a compressed, metal seal.
Also, in many industrial packaging situations, oil or grease may be accidentally or purposely applied to the strap. In any case, application of a compressed seal about such strap segments to effect a friction joint therebetween may not establish a tight enough joint that will hold under the tension applied to the strap. Even if the seal at first securely grips the overlapping strap segments, the strap segments may start to slide within the seal over a period of time or when subjected to vibration and other shock loading conditions.
To overcome these problems of seal/strap slippage, a number of seal modifications have been developed. The U.S. Pat. No. 3,089,233 to Meier discloses a seal blank which is coated on the inside with relatively hard, small grit particles. When overlapped end portions of a strap are secured together by the seal, the particles are embedded in the adjacent faces of the strap end segments and hold the strap end segments against relative longitudinal movement. The U.S. Pat. No. 3,237,256 to Young also discloses a plastic strap seal wherein grit material is secured to the inner, strap-contacting surfaces of the seal.
Though the above-discussed grit-type seals function satisfactorily to cut through layers of wax, oil, or paint on strapping and form a secure joint for some types of strap, the grit on the seals poses a problem since some of the particles of grit tend to become detached from the seal and are then carried, or fall, into the tool or machine used to compress the seal about the overlapped strap portions. Eventually, a build-up of grit within the tool or machine causes operational problems. Thus, it would be desirable to provide a gritless seal free of any sources of particulate matter which could enter a seal-applying machine or tool and have deleterious effects.
Such an improved, gritless seal should function to provide relatively high joint strength when used with the harder thermoplastic strapping materials, such as polyester strapping material.
Further, it would be desirable if such an improved seal could be adapted for use with the relatively thick, conventional, polyester strap having a width of about 5/8 inch and a thickness of about 0.033 inch.
Additionally, it would be beneficial if such an improved seal could be manufactured relatively inexpensively by means of a readily controllable process capable of producing a readily controlled seal configuration with more controlled strap-gripping characteristics.
It would be desirable to provide such an improved seal with a design susceptible of embodiment in a form that could reduce or minimize strap stress concentrations and that could provide a more uniform gripping engagement of the strap material.
It would be beneficial if such an improved seal could be provided with a design that exhibits little or no "directionality" relative to the tension forces in the overlapping strap portions. That is, it would be desirable to provide gripping structures on such an improved seal that exhibit substantially the same holding power in opposite directions along the length of the strap.
Finally, it would be advantageous if such an improved seal had a strap-engaging structure that would accommodate the heat treatment of the seal or the coating of the seal with hot metal or glass spray.